Conformation of the cyclic tetrapeptide dihydrochlamydocin. Iabu‐<scp>L</scp>‐Phe‐<scp>D</scp>‐Pro‐<scp>L</scp>X, and experimental values for 3 → 1 intramolecular hydrogen bonds by X‐ray diffraction

Abstract

Abstract Chlamydocin, Iabu‐ L ‐Phe‐ D ‐Pro‐ L X, is a naturally occurring cyclic tetrapeptide that exhibits high cytostatic activity. The conformation of the peptide ring, as well as the stereo configuration in the vicinity of the epoxide ring, have been established by a single‐crystal X‐ray study of dihydrochlamydocin: C 28 H 40 N 4 O 6 ·H 2 O. It crystallizes in the monoclinic space group P2 1 with a = 12.616(6) Å, b = 12.355(6) Å, c = 9.442(5) Å, and β = 99.5(1)°. The structure was solved by the symbolic addition procedure for phase determination followed by the tangent formula method of phase refinement. This structure represents the first cyclic tetrapeptide in which all four peptide units have been found in the trans conformation; however, each peptide unit is significantly nonplanar with ω angles deviating by 14–24° from the ideal value of 180°. This molecule contains two intramolecular 3 → 1 hydrogen bonds and experimentally determined parameters for these seven‐membered turns are presented.